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Timelike nucleon electromagnetic form factors: All about interference of isospin amplitudes

A striking feature of the timelike nucleon electromagnetic form factors, investigated in $e^+e^- \to N\bar N$ annihilation reactions, is the modulation by local structures of small magnitude and oscillatory form, showing up above $N\bar N$ threshold. Starting from an isospin decomposition of the proton and neutron form factors it is shown that such structures are the natural consequence of the interference of a large and a small amplitudes, resulting in a sinusoidal behavior as a function of the "invariant energy" if the relative phase shift varies with energy. Thus, periodic oscillations superimposed on a smooth background will be observed. In this scenario, an equal size of the modulation for neutron and proton discovered by recent BESIII data evidently implies the particular isoscalar or isovector nature of these local structures, or their orthogonal interference, hence specifies their origin as excited vector mesons whose widths are tied to the modulation frequency.We clarify that the phase difference of modulation between neutron and proton as BESIII data found, but not the modulation itself, is the evidence of an imaginary part of the timelike nucleon electromagnetic form factors, which is associated with the rescattering processes.